Optical receivers, that include photodiodes, amplifiers and other components, convert optical signals into corresponding electrical signals in many types of optical systems. Because this conversion, or photodetection, often involves processing low magnitude signals, periodic zero calibration of the optical receivers is performed to accommodate for offset voltages of the amplifiers that influence the accuracy of the conversion. Known zero calibration methods rely on a dark signal measurement at the output of the amplifier. During the dark signal measurement, an optical signal, normally applied to the photodiode during photodetection, is interrupted by extinguishing the optical source supplying the optical signal or by redirecting optical fibers supplying the optical signal, so that optical signals do not illuminate the photodiode. Extinguishing the optical source has the disadvantage of imposing time delays in the zero calibration of the optical receiver because of the time required for the optical source to stabilize once the optical signal is re-applied to illuminate the photodiode. Redirecting optical fibers supplying the optical signal to the photodiode has the disadvantage of introducing variations in the coupling characteristics of the interface between the photodiode and the optical fiber that decrease the accuracy of the conversion. There is a need for a zero calibration technique for an optical receiver that does not rely on interrupting the optical signal applied to the photodiode. This need is met by the zero calibration system and method constructed according to the embodiments of the present invention.